Optical time domain reflectometer (OTDR) is a measuring device based on the backscatted or back-reflected signals. It's the most important tool for locating the fiber faults and estimating the fiber's length and overall attenuation, including splice and mated-connector losses. It can measure fiber by a non-destructive means and has a wide range of application in the field of optical fiber industry like optical fiber research and manufacturing, optical fiber networks installation and maintenance, playing a very important role in the fiber industry.
Normally OTDR comprises four units: the laser generating unit, the laser receiving unit, the signal processing unit and the display unit, in which the first three units are critical parts, representing the core technology of OTDR. The conventional OTDR injects a short optical pulse emitted from a pulse laser to an optical fiber under test. It also extracts, from the same end of the fiber, the optical signal that is backscattered and back-reflected from points in the fiber where the index of refraction changes. The power lever of the return pulses can be measured as a function of time, and the fiber condition is represented as a function of distance.
While the drawback of such an OTDR technique is that its resolution is limited by the width of the optical pulse, which is generated by pulse laser as light source. For example, to achieve a resolution of 100 m, pulse width should be less than 1 μs. The dynamic range of the OTDR depends on the amount of energy that is launched into the fiber and is limited due to the available laser pulse peak power. Increased pulse width reduces the resolution of the pulse-based OTDR. To enhance the dynamic range, the resolution will be reduced accordingly, and vise versa, which has become an inextricable problem for the pulse-based OTDR.
In the related art EP0269448 and JP9026376, an improved correlation OTDR was developed to overcome the tradeoff between resolution and dynamic range of the pulse-based OTDR. In correlation OTDR, light source is acted by a low-power continuous-wave semiconductor laser modulated with pseudorandom code, and the reflection point is located by correlating the backscattered light with a delayed code sequence. By enhancing the total energy of probe light with increasing code length, dynamic range can be enlarged, without losing resolution for given pulse width. This method can improve the dynamic range and spatial resolution significantly. However, fail to make full use of the advantages of the correlation technology; the resolution is still limited by the bandwidth of the electrical codes. The unambiguous detection span is also limited due to the repeat waveform of pseudorandom codes. Also, complex devices, such as pseudorandom codes generator and encoding and modulation circuits, are needed.